The present invention relates generally to hand held food mixers and more particularly to an electric motor and speed control circuit for such a mixer.
An electric hand-held food mixer typically involves a housing having a handle and containing an electrical motor and gearing linking the motor to counter-rotating receptacles for removable beaters. A line cord for connecting the mixer to the household electrical current supply is provided. Multiple speed selection is usually provided in the form of a thumb operated switch.
One problem with universal motors used in food mixers is that it is a characteristic of such motors to decrease in speed in response to an increase in load, and to speed up in response to a decrease in load. As a result, when the beaters of the mixer are introduced into the food to be mixed, the speed of the mixer often slows to an undesirable level. In an attempt to compensate for this action, the operator will often select a higher speed setting with the speed select switch which causes the mixer to return to an acceptable speed under load. The difficulty arises when the beaters are retracted from the food or the load is otherwise suddenly reduced, such as by adding liquid to the food being mixed. Upon reduction in load, the mixer motor suddenly speeds up, which can result in food being flung at high speed in undesired directions.
The problem underlying the present invention is to provide a simple, economical variable speed motor and speed control therefor for use in a hand-held mixer which provides good speed regulation at any selected speed setting in response to varying load conditions.
Universal type electrical motors have found wide acceptance for use in hand-held food mixers, with a variety of speed control circuits. One common type of speed control circuit involves chopping the 60 Hz AC voltage waveform applied to the motor so that current is delivered to the motor over only a portion of the current cycle. Lower speeds are achieved by chopping out a greater portion of the waveform, such that the average applied voltage is considerably less than the average of the full sinusoidal form.
Chopping of the waveform can be achieved through switching type circuitry. A triac semiconductor device is sometimes used as the heart of such switching circuitry. A triac is a three terminal device with which relatively large amounts of AC current can be switched between two main terminals by a very small trigger current in a third trigger terminal. Triacs revert to a non-conducting state upon reversal of current flow through the main terminals, which makes them especially useful in controlling alternating current circuits. The triac can be turned on with appropriate control circuitry at a selected phase angle in each half cycle.
It would be desireable to provide a speed control circuit for an electrical motor for use in a hand-held mixer which utilizes proven current-chopping speed control with a universal motor, but wherein speed regulation in response to load changes is improved.